The only method I know of is to get it directly from a reputable source. In the US, I would not buy this from Amazon. I would trust some of the big box stores if I purchased it in the store. That said, you can get scammed anywhere.
I don’t know a good way to verify it’s genuine before purchase and I’m not familiar with purchasing it in your region. They do sell it directly on the US Canon site and list other online retailers. That’s probably the route I would go if you want to be safe. Not sure about purchase and shipping to Vietnam, you may be able to purchase online from somewhere nearby that is an “authentic canon retailer” or however they would phrase it.
It’s really going to depend on your use. Feel free to try 90 first. I default to the highest because it is relatively cheap compared to the rest of the equipment to just get the best. This will affect how quickly you can take pictures back-to-back.
I recommend taking shots in RAW so you can edit them properly later (Canon should include basic editing software for free). Most programs cannot read RAW images, but there are various free programs that can. I’m pretty sure GIMP can, but I’ve never tried using GIMP for this. There should be an option to take images in RAW+jpeg if you want an immediate jpeg copy in addition to RAW.
You can shoot directly to jpeg, but I recommend keeping RAW copies of at least the images you like so you can edit them later if you don’t start out editing. RAW files are much larger than jpeg, but are meant to be edited and then exported as a jpeg or something else afterward.